In California, heavy-duty vehicles are major contributors to global warming emissions. Biomethane, a potential substitute for natural gas derived from the decomposition of organic matter, has been proposed as a lower-carbon alternative to diesel in heavy-duty vehicles—but how much of a dent can it make?

The good news is that biomethane offers a pathway to reduce global warming emissions from landfills, wastewater treatment centers, and dairies, which all generate significant amounts of methane that goes largely uncaptured today.

The bad news is that there’s a limited amount of biomethane from waste compared to existing energy and fuel use in California—and we shouldn’t encourage creating more waste than we already do.

Meeting California’s diesel demand would require all of the potential sources of waste-based biomethane in the United States. And biomethane from waste in California could meet just 3 percent of the state’s demand for natural gas. Increasing biomethane production will require expansion of natural gas infrastructure and improvements in waste management.

Given the limited amount of biomethane, a large scale shift to natural gas heavy-duty vehicles could increase California’s reliance on natural gas and the local impacts of extracting and delivering it, ultimately undermining the state’s climate goals.

How does biomethane compare to natural gas?

Biomethane and natural gas have similar chemical compositions. They consist primarily of methane–a potent global warming gas—and can be used interchangeably. At the tailpipe of a vehicle, they produce the same emissions.

But, biomethane and natural gas do differ in their source, which results in different "upstream emissions." This means that although biomethane and natural gas combust in similar ways, their overall ("life cycle") emissions are different.

Natural gas is a fossil fuel that comes from ancient (fossilized) plant and animal matter found beneath Earth’s surface, and has to be extracted.

Biomethane results from the decomposition of plant or animal matter, such as that found at dairy farms, landfills and wastewater treatment facilities. Upstream emissions are lower for biomethane than natural gas because its use offsets emissions from waste.

Biomethane’s emissions benefits depend on how it’s used

While biomethane can directly replace natural gas in heavy-duty vehicles, it can also be used to generate electricity or hydrogen for battery or fuel cell electric vehicles (such as the battery electric bus shown here). This results in greater carbon emissions reductions due to the higher efficiency of electric vehicles compared with conventional vehicles. Photo: Foothill Transit.

Biomethane generates lower life cycle global warming emissions than natural gas in a vehicle, and it results in even lower emissions when used to make electricity or hydrogen for battery or fuel cell electric vehicles. This is due in large part to the higher efficiency of electric vehicles.

Likewise, using biomethane to power battery and fuel cell electric vehicles results in lower emissions of smog-forming nitrogen oxides (NOx) than using the biomethane in a compressed natural gas (CNG) vehicle.

While biomethane should be used to displace consumption of fossil fuels, wide-scale adoption of battery and fuel cell electric vehicles is ultimately needed to meet climate and air quality goals.

Life cycle NOx emissions from transit buses, by vehicle and fuel type. The lowest life cycle emissions of nitrogen oxides (NOx) from biomethane results from generating electricity or hydrogen for use in battery or fuel cell electric vehicles. Source: Chandler, Espino, and O’Dea 2017.

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